1. Field of the Invention
The present invention relates to an etching method. More particularly, the present invention relates to an etching method capable of reducing critical dimension (CD), and to a method for forming a contact opening that utilizes the etching method.
2. Description of the Related Art
As the integration degree of IC is always required to be higher, the dimension of semiconductor devices unceasingly gets smaller. In the prior art, the miniaturization of pattern pitch in IC fabrication is mostly made by enhancing the lithographic resolution. However, high-resolution lithography techniques are more difficult and expensive due to the limitations of optics.
Instead of enhancing lithographic resolution, the critical dimension of pattern after etching can be reduced alternatively by modifying the etching process. In the modified etching method, a patterned photoresist layer with an after-development-inspection (ADI) critical dimension (CD) is formed with a conventional lithography process, and then a polymer-rich and low-power recipe is used to etch the target layer in a tapered manner so that the after-etching-inspection (AEI) CD is smaller than the ADI CD. However, since an opening in the photoresist layer and the anti-reflection coating (ARC) is easily expanded in such an etching method, the CD of the corresponding opening in the etched layer cannot meet the requirement. On the other hand, the above etching method has a limitation on the extent of CD reduction and may cause a striation effect, upon which the top-view profile of the openings is much changed so that adjacent openings overlap with each other to cause bridging between adjacent contact plugs. Therefore, the reliability of the devices is lowered.